Wavelength tunable semiconductor lasers based on grating reflectors are promising devices for future telecommunications systems requiring an extended tuning range, e.g. for Wavelength Division Multiplexing (WDM) applications. While quasi-continuous electronic tuning over a range of 100 nm is possible, the tuning range of a single Distributed Bragg Reflector (DBR) grating is limited to about 10 nm.
The material refractive index of a semiconductor laser can be changed by current injection and thus the reflection peak of one set of gratings can be tuned over a certain range of wavelengths .DELTA..lambda. of 10-20 nm. If the multiple reflection peaks of multiple m sets of gratings are spaced apart by roughly .DELTA..lambda. in one DBR mirror, the corresponding peaks in the other DBR mirror are spaced slightly differently. Superimposed gratings provide several reflection peaks in the resulting diffraction spectrum or reflection spectrum. Superimposed gratings have therefore been used in a DBR region of a tunable laser to extend the tuning range of the lasing wavelength to several times that of a conventional single grating laser. Then the total tuning range is extended to m.DELTA..lambda. which is broad enough for many applications. The total tuning range would be limited only by the material gain spectrum bandwidth, e.g. that of conventional 1.55 .mu.m laser.
In efforts to improve the tuning range of DBR lasers, various types of multiple wavelength filters with comb-like reflectivity gratings have been demonstrated. These gratings provide a Fourier spectrum comprising an array of peaks. The simplest, most direct way to realize a grating structure with several peaks in the Fourier spectrum is to superimpose a few different gratings in a waveguide.
Superimposing gratings in a photosensitive glass is reported in a publication by V. Minier et al., entitled "Diffraction characteristics of superimposed holographic gratings in planar optical waveguides" , in IEEE Photonics Technology Letters, Vol. 4, no. 10, October 1992. However, this technique is not practical to obtain superposition of gratings within a semiconductor material, where a grating is created by a photo-lithographic technique. In the latter multiple patterning and etching process steps are required.
For example, an analog superimposed grating (SG) structure from France Telecom is described in PCT Patent application WO 96/11416 published Apr. 18, 1996 entitled "Optical filter for a plurality of guided wavelengths". This structure includes a continuous grating in which every basic part comprises a plurality of gratings with periods corresponding to the various respective filter wavelengths. The several gratings are inscribed in a stack one above another in a single guide layer, or in a stack of guide layers. Earlier work on superimposed gratings was reported by one of the present inventors, in a paper by Minier et al., entitled "Superimposed phase gratings in planar optical waveguides for wavelength demultiplexing applications', in IEEE Photonics Technology Letters, vol. 5., No. 3 March 1993, which describes a coupled mode analysis of superimposed holographic gratings.
A comb-like reflection spectrum is also provided by a sampled grating, e.g. a structure comprising sections of grating alternating with grating free sections. However, a sampled grating is intrinsically unsuitable for equal amplitude reflections at multiple wavelengths, or when it is required that DBR zone is reasonably short, with a modest level of loss.
A superstructure grating may be constructed to provide a comb-like spectrum. In a grating of this type of structure, a parameter of the grating is modulated along the length of the grating, for example, in a linearly chirped grating where the pitch of the grating is modulated linearly along the length of the grating. Other known structures include a linearly stepped chirped grating, or a quadratic stepped phase chirped grating. These structures provides improved performance, but, patterning and fabrication of these grating structures is much is more complex. As described by NTT in IEEE J. Quantum electronics vol. 29, pp. 1817-1823, 1993; and IEEE J. Quantum Electronics, vol. 32, pp. 433-441, 1996, a superstructure grating (SSG) provides near maximum effective coupling coefficient for a given depth of index change, and permits adjustment of individual reflection peaks amplitudes. Nevertheless, the NTT SSG relies on super high precision (.about.1 nm) photolithography, requiring specialized processing equipment.
The SG and SSG structures mentioned above are believed to represent the best performance known to date, being suitable for broad &gt;80 nm continuous tuning, and current efficient wavelength tuning. Nevertheless complex fabrication is required. For example the analog Superimposed Grating reported by France Telecom requires multiple etching steps, and multiple deposition and regrowth steps. Consequently manufacturing is relatively expensive and good reliability and yield require careful control of multiple process steps. On the other hand, super structure gratings rely on specialized precision lithography equipment.
Consequently there is a need for alternative grating structures which provide the required performance characteristics, preferably using lower cost, straightforward fabrication processes.